The present invention relates to an electronic ignition timing control system for electronically determining the ignition timing of an internal combustion engine (hereinafter referred to as an engine).
With a distributor of the type which has heretofore been used widely for timing the ignition of an engine, the relative positions of the elements in an assembly of a cam and electric contacts employing points are changed in accordance with the engine speed and the intake vacuum which are respectively detected by a centrifugal governor and a vacuum advancer so as to generate an ignition signal at the ignition timing that suits the engine operating conditions.
However, the distributor of the above type is disadvantageous in that since all the controls are accomplished mechanically, the accuracy of the resulting ignition timing cannot be fully satisfactory and it is also difficult to obtain an ideal ignition timing characteristic, thus giving rise to problems from the standpoint of exhaust emission control which has become an issue.
An electronic ignition system has been proposed in which the preset advance angle is divided by the speed of an engine and converted into a time so as to time the ignition of the engine. A disadvantage of this type of system is that since the engine speed is measured, a certain time is required for measuring the engine speed and consequently the value of the measured engine speed represents the average engine speed over the measuring time, thus causing an error when the engine speed changes during the measuring time. Another system has been proposed in which in consideration of this disadvantage, the ignition of an engine is timed by detecting the slits formed in the circumferential portion of a disk and each corresponding to a predetermined crank angle. In practice, however, if each slit corresponds to 2.degree. in crank angle degrees and the disk is mounted on the crankshaft, 180 slits must be formed in the disk and this system will be considered impractical in consideration of the available slit forming capacity and the ability and durability of the available slit detecting sensors.